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Imagine you’ve been tasked with clearing a disused shed for an upcoming project. Overgrown and undisturbed, nature has fully made itself at home and after sweeping away the leaf litter you try to tackle a strange fur that's growing on walls. To your surprise, a surreptitious poke or two sends the ball of fuzz scurrying in every direction as it dawns on you: this is no fluff ball, but an enormous swarm of daddy longlegs.
This strange and unsettling gregarious behavior is one employed by Opiliones, a group of slender, spider-like creatures known as harvestmen. They differ from cellar spiders, also sometimes called daddy longlegs but sit within the genus Physocyclus (and aren’t as venomous as everyone thinks). Both daddy longlegs varieties are within the class Arachnida, but harvestmen aren’t within the order Araneae, which contains spiders.
That said, it doesn’t stop these creepy crawlies from being any less many-legged and twitchy, and their anatomy creates an unusual effect when they swarm en masse, forming enormous fuzzy beards that move as one across buildings. When irritated, harvestmen will pulse in what’s believed to be a defensive strategy to ward off predators. If I approached a furry wall and it started pulsing at me I’d probably think I’d smoked one too many Jeffreys.
For Halloween, the Glacier Bay National Parks Service reshared the display put on by a swarm of Opiliones a couple of years ago as their administrative office became coated in a mobile layer of fuzz made up of the swarms' many, many, long legs. It might look a little counterproductive if you’re someone who values personal space, but this crowding behavior actually helps to keep them safe. Beyond looking freaky enough to ward off predators, Opiliones are a stinky bunch and by pooling their offensive odor they make themselves unpalatable to even the boldest of enemies.
So, if you find yourself faced with a building that appears to have sprouted a beard, take heed. You might not want to mobilize the mob.
